The present invention relates to an anti-lock control apparatus for preventing locking of automotive wheels during braking thereof.
Generally, in an anti-lock control apparatus for automotive vehicles, to maintain steering ability and running stability during braking, a control unit including a microcomputer controls brake hydraulic pressure. This brake pressure control reduces braking distance.
During operation of the anti-lock control apparatus, a control mode for brake hydraulic pressure is determined on the basis of an electric signal indicative of a wheel speed. This wheel speed is detected by a wheel speed sensor. According to the control mode, a hold valve, which is normally opened solenoid valve, and a decay valve, which is a normally closed solenoid valve, are selectively opened and closed to increase, hold constant and decrease the brake hydraulic pressure.
In such anti-lock control apparatus, as disclosed in the U.S. Pat. No. 4,929,035, three-channel anti-lock control is widely used such that the brake hydraulic pressure for left and right front wheels is controlled to be increased and decreased on the basis of left and right wheel speeds through first and second control channels independent of each other, respectively, and the brake hydraulic pressure for left and right rear wheels is controlled to be increased and decreased on the basis of a common wheel speed which is a selected lower one from the left and right wheel speeds through a third control channel.
However, such front-independent and rear-select-low three-channel anti-lock control in which the brake hydraulic pressure for left and right wheels is controlled on the basis of a common select-low wheel speed through a common control channel, suffers from a problem in that if the vehicle moves on so-called split .mu. road in which the surface of the road differs in the coefficient of friction between the left and right wheels, the speed of the wheel on the higher friction coefficient part of the road surface drops more than that of the other wheel on the lower friction coefficient part of the road surface after the start of the increase in the brake hydraulic pressure. This causes the decrease in the brake hydraulic pressure to be started through the common control channel for both the left and right wheels. Therefore, the decrease in the pressure is started for the wheel on the lower friction coefficient part of the road surface before the speed of the wheel is not decreased enough. Besides, the increase in the brake hydraulic pressure for both the wheels is not started until the speed of the wheel on the higher friction coefficient part of the road surface recovers to the vicinity of the speed of the vehicle. As a result, the brake hydraulic pressure for the faster wheel on the lower friction coefficient of the road surface is likely to become insufficient.